Demystifying the Sony a7R shutter operation

It’s been clear to anyone testing the Sony alpha 7R that its shutter is a different animal from what DSLR shooters are used to. I’ve been reporting on what that shutter does to pictures of test targets, of oscilloscopes, and of real subject matter. But what do I know about what goes on inside the camera when you press the shutter release? Not much. Time for a different perspective.

Mike Collette is a self-taught electronics engineer who invented the large-format digital scanning back in 1992, and founded Better Light, Inc. (www.betterlight.com) to develop and manufacture these high resolution digital capture devices. In addition to being a fine engineer, Mike has an unusual ability to explain complex technical issues to people who think a logarithm is a line dance. He’s also an accomplished photographer and a nice guy. Check out his work at www.betterlight.com/gallery.

While I’ve been using an oscilloscope to look at the effects of the Sony a7R’s shutter slap on images made with the camera, Mike’s been doing some great detective work on the causes of that shutter slap – or, more properly, as you’ll see, those shutter slaps.

I’ll let Mike tell you about it himself. In order to fully appreciate what he’s done, keep in mind that a loudspeaker contains an electric motor: it turns current into linear motion. Almost all electrical motors can be run backwards, and then they turn motion into current. When they are doing that, they’re called generators.

“Today I cannibalized an old computer external speaker to make a pseudo-accelerometer so I could measure the external vibration when the Sony a7R shutter is fired. Attached is a photo of the simple test setup. On the other end of the probe is a fancy digital oscilloscope that lets me record and expand whatever the speaker “felt”, and even transfer screen shots to my computer via USB cable.

“My test setup utilizes a high-tech cylindrical fiber tube (a rolled-up Post-It note) to transfer vibration from the camera body to the speaker’s voice coil (about the same diameter as the tube). The speaker voice coil is only responsive to vertical motion (in this setup), but horizontal motion of the camera body might “rock” the top end of the tube and cause some vertical displacement of the bottom end anyway. Since the focal plane shutter travels vertically in the Sony, I figure vertical body motion would be more important, and it’s also easier to “mount” the camera on its flat bottom than on either side.”

Here’s Mike’s setup:

Here’s the ‘scope image with Mike’s annotations showing what’s probably happening during each material segment of the waveform:

The annotated waveform image above is way too small to see in the blog window. However, if you click here, you can see it at whatever resolution your browser supports, up to 3085 pixels wide. If that’s not big enough, you can right-click this link and save it on your computer (Firefox and Chrome: “Save link as…”, Internet Explorer: “Save target as…”), you can look at it with any image program that can handle PNG files. Photoshop is one such program.

Mike has some further comments:

“I don’t know exactly when Live View actually ends, but I’m guessing it stops before or as the shutter starts closing. I don’t know when Live View actually starts either, but I’m pretty sure that this won’t be before the shutter is open again [OK, Mike, we get it].

“I also don’t know exactly when the shutter rewind/cocking actually begins; there is some vibration before the starting points I marked that could be related to this action, too.

“What the waveform does indicate is that the shutter doesn’t “slap” closed initially, but rather gets rewound closed. Note that there is a slight wiggle after the second shutter rewind/cocking (before the first curtain opens again), so the rewind action is creating some disturbance that might still justify having a longer delay before the exposure begins. But most of the disturbance is coming from the first and second curtains. There is an initial “recoil” (the downward swoop) as the curtain is accelerated into motion, and then a “slap” (the upward spike) when the curtain abruptly decelerates. Both of these forces impart energy that can contribute to camera shake, although only the initial recoil of the second curtain matters, since the exposure is over before the second curtain slaps.

“In fact, the one-two punch of the first curtain recoil (down) and slap (up) produces a nice little wiggle that could tickle any resonance in the camera (a mass) and its support (a spring, more often than we would like). The big disturbances in the waveform following each shutter motion are caused by the resonance of my test setup, which had the camera and 55mm FE lens sitting on big blocks of dense closed-cell foam — certainly not a typical mounting arrangement, but good for allowing the camera body to move slightly so the speaker could “feel” it better. At least the foam is fairly well-damped so the camera settles down within a few wiggles. These shutter-induced wiggles will be considerably different in a typical setup, especially with a long lens adding its mass to the camera, and perhaps some springiness as well if the lens mounting foot isn’t stiff enough. A dead weight as proposed by Joe Holmes also adds mass to the camera that can help de-tune any resonance in the support system, or exaggerate a resonance, if we’re not careful.

“At shorter exposure times, the second curtain recoil/slap moves closer to the first curtain recoil/slap, until at 1/160 second (the fastest flash sync speed for the a7R), the second curtain starts closing almost immediately after the first curtain finishes opening, leaving just enough time for a flash to fire while both curtains are completely open, superimposing the second curtain’s recoil (downward swoop) on the first curtain’s slap (upward spike). While this overlap might help reduce the effect of these overlapping opposing forces, the first curtain recoil is unaffected, and any shake from this action represents the majority of what the sensor sees during such “short” exposures. Even shorter exposures have the second curtain starting to close before the first curtain has finished opening, so the two curtains chase each other across the sensor leaving a variable-width slit that determines the effective exposure time. This also means that both curtain recoils start overlapping and perhaps exaggerating any camera shake as the exposure is ending.

“If the Sony has already opened its first curtain to allow Live View, the only way to close this would be to rewind/cock this curtain again (note that the second curtain is already open/cocked, so the first “rewind/cocking” interval only has to rewind/cock the first curtain). After the exposure completes, both curtains are rewound/cocked again, probably keeping the sensor in complete darkness during this operation. Finally, the first curtain is fired open to allow Live View again.

“I suspect that during multiple exposures, the second opening of the first curtain and the initial rewind/cocking are omitted between exposures, and the shutter only needs to be rewound/cocked during the image readout portion following each exposure.

“Rewinding/cocking these shutter curtains in just over a tenth of a second is probably being accomplished by a fast electric motor, and if the curtain actuating springs utilized bistable “flippers”, most of the rewinding/cocking time would be winding up the springs, and near the end of this interval the curtain(s) would “flip” to their cocked states and produce the high-frequency “chatter” that looks similar to the shutter slap signatures in the waveform.”

I have been agitating for a firmware change to provide delay between the closing of the shutter when the release is pressed, and its opening to take the picture. Based on what he sees with his test setup, Mike doesn’t think this is a fix for the camera’s shutter slap:

“I don’t think much will be gained by asking Sony to delay the start of exposure after the shutter closes, although this shouldn’t be detrimental in most applications, either. The best way to address this situation is by paying careful attention to the camera mounting/support system. Every adapter, screw, mounting plate, quick-release, clamp, bracket, rotary head, etc. needs to be considered as a potential source of instability/elasticity.”

He puts the a7R’s problems into perspective:

“This problem certainly isn’t new to digital photography, but as sensor resolution increases (and as camera bodies shrink), camera shake will continue to be a potential source of image degradation.

“As much as I prefer electronic solutions to just about everything, there isn’t much better than a simple torsional spring to store and release energy quickly and with minimal inertia. I’m also guessing that most focal plane shutters behave similarly — when rewound/cocked, the first curtain is closed and the second curtain is open, and when fired, the first curtain opens followed by the second curtain closing. Exposures shorter than the transit time of a curtain across the image plane are effected by releasing the second curtain before the first curtain has fully opened, so the two curtains form a variable-width slit that travels across the image plane. The fastest flash sync speed is determined by the transit time of a curtain across the image plane; both curtains have to be fully open when the flash fires, and then the second curtain can begin closing.”

Mike’s experiment, and more importantly, his interpretation of the results, have made what’s probably going on in the a7R’s shutter much clearer to me, and has suggested some further experiments on my part. Thanks, Mike.

Comments

Thank you for the article. Interesting explanation but I am not sure I fully understand the implications. If the shutter vibration problem is the result of increased sensor resolution and size/weight camera body decrease will adding weight to the bottom of the A7r (i.e. a metal base that just adds weight) improve the shutter vibration? If so, will it work equally if the camera is hand held (I have noticed the shutter vibration even while hand holding the camera)?

Joe Holmes has done a lot of work on this, and he’s found, for a few lenses, that adding 24 oz to the camera in the form of a weight helps a lot at 1/100. Adding weight to the camera will change the resonant frequency, and will also change the amount of movement the camera makes in response to a displacement of mass (the shutter) inside the camera. Forgetting frequency for the moment, the amount of camera displacement is proportional to the ratio of the moving mass to the mass of the camera/added weight. Putting frequency back into the mix, lowering the resonant frequency will probably lower the worst shutter speed for any camera/lens combination, and might actually make things worse at some shutter speeds.

I’ll be making a post that relates to this point later today.

The only way for you to get a definitive answer to your question is to print a copy of the ISO 12233 chart and do some testing. Unfortunately, it is very difficult to do repeatable tests of handheld performance; focus issues affect the results, as does the steadiness of your hand. You will need to take many images and do some statistics on the results.

Your last comments, about the effects of adding weights to the camera, appear from my recent experiments with better parts in hand for testing, to be quite correct.

Now that Mike Schultz and I both have a first generation of finished deadweights for attaching with small RRS clamps to the base of the A7R (right side for a vertical image), and some other new parts for testing various other support options, including a modified long lens support setup using RRS parts but not those in their standard setup, we have been running a great many experiments.

One of my key test configurations is to attach my Canon 70-200 f4L (non-IS) lens to the A7R with the only adapter I have which is working so as to make shooting possible — a simple Vello adapter. I don’t have a lens foot for this lens. I have been running the tests with the lens pre-set to f/8 using a Canon body to stop the lens down, then remove the lens while holding down the DOF preview button.

I set the lens to 200 mm for most of the tests, and try various approaches to suppressing motion blur artifacts in the images. What we have seen is that the worst shutter speed for a given setup can be (with lesser mass and/or shorter pendulum setups) 1/100th of a second (as seen in experiments a couple of weeks back) or longer, peaking somewhere in the 1/50th to 1/20th range, when the lenses are physically longer.

So, some setups can look great at 1/100th but lousy at 1/30th and thereabouts. One such setup was Mike’s 70-200 Nikkor f4 VR connected by its Vello collared lens foot, and with a deadweight connected to the camera. So this setup has been seen to still have issues (at the slower shutter speeds), despite our earlier hope that it would simply work fine. (It is a big improvement though.) Connection of the tripod directly to the camera is important. As is preventing the lens from wiggling around in the wind. (Our tests are entirely indoors with zero external vibration sources.)

The bottom line of my own tests is that only by doing everything I’ve come up with to improve the situation altogether, can I just barely get excellent sharpness across all the shutter speeds from 1/200th to 1/2 second.

The samples I showed you a couple of days ago at 1/100th, 1/50th and 1/25th were from a result which combined all but one of the things I have found thus far to improve the situation, and I would characterize them as pretty good, but not really sharp, not crispy if you will.

Since then I upped the mass of the deadweight by 6.4 or 6.9 ounces (one or the other, not both) for a total of 16.8 + 6.x, excluding the mass of the L-plate from RRS. By the way, the fit of the L-plate is superb, from the careful examination I did of its base plate and the way it fits the bottom of the camera. Note that it is very important to remove the UL sticker from the base of the camera (likely a US or North America only sticker) because it sticks out about 0.004″ and creates a plateau which totally messes up the beautiful fit of the L-plate to the camera body. This fit is complex because it involves two major 45-degree bevels, front and rear edges. And the tripod screw location as well. All seem to be just right, to at least 0.002″ tolerance and I think 0.001″ or so.

This shutter is a beast if you’re trying to get sharp results with a vertical picture at 200 mm and all shutter speeds. If you’re doing lots of other things, it’s not an issue (for all you over-generalizers out there…). Nevertheless, I am still hopeful that the camera will be usable with a range of lenses between as short as you want, and long out to 200 mm with the camera vertical, if one does several things to stiffen up the camera’s relationship with the tripod and to provide about the right mass for the shutter to bounce against.

In some cases (e.g. where the camera is directly connected to the tripod) horizontal images turn out much better, as the shutter finds it quite difficult to move the camera up and down in that case. But indeed, as has been theorized by various people and as we can feel with our hands, if the camera + lens assembly is connected amidships, the shutter may have an even easier time moving the camera up and down than left to right, so a horizontal capture may be more problematic. But if we can connect the camera directly to the tripod, then the problematic situations are apt to be limited to verticals with lenses of say, 150 and up, but that’s a matter of degree and one shouldn’t assume that my mention of 150 is some kind of dividing line in a general sense. Some of us have even seen 55 mm lenses reveal shutter shake artifacts. Speaking of which, there are two classes of artifacts, if I may suggest such a classification: first, the obvious uni-axis blur which one associates with motion blur in camera systems, and second, a more radially symmetrical blur. The latter is a more subtle effect and may account for all of the observed degradation thus seen with the 55 lens. It’s much harder to spot as shutter shake just by looking at the image, because the image simply looks a little less sharp than it otherwise would have.

Also, my testing does repeatedly show that if I go to weights totaling around 32 ounces, the situation gets considerably worse than if I have, say 16.8 or 24 to 25 ounces attached to the camera (excluding the 4 oz mass of the L-plate). On the other hand, Mike did one somewhat questionable experiment (means of attachment of the weights was not positive) where adding 40+ ounces was better than adding 24. So, we have therefore seen some, so far vague, evidence of even a double-humped response, suggestive of harmonic issues being significant, as oppose to simply mass-damping issues. Then again, even a single-hump would suggest harmonic issues (i.e. as you increase the weight it doesn’t simply get better and better, as I think both you and Mike Collette wrote, in the quoted material above).

I am in need of refining the differences between just increasing the mass and inadvertently providing torsional resistance because that mass is long (a nodal slide and a similarly elongated part were what I used to most successfully add mass to my 16.8 oz weight), as I try to figure out the optimal mass and dimensions for a second-generation deadweight. It’s looking like I’ll want one which is 24 or 25 ounces, and probably again stainless steel, though I’ve been carefully considering other metals once again, just in case something else might be more compact without the toxicity of lead, the cost of silver, or the oxidizability of tungsten, for example. Stainless is easy to get and work with and has a specific gravity of approximately 8.0. A denser material such as tungsten might actually be less desirable for day to day use, just because of the way it will feel in your hands.

So far, my 16.8 ounce weight seems quite friendly and easy to use with my various setups and lenses. The finished weight looks quite nice too.

Oh, one more thing: Mike Schultz and I figured out that we could stiffen up the L-plate a lot when it’s in the vertical position by deliberately clamping the camera’s left strap triangular connector between the L-plate and the body. This kludge stiffens the setup pretty dramatically and also helps the image blur a goodly amount. This, together with direct connection to the camera, plus having a deadweight of roughly the ideal mass, plus no soft connections, plus, I think, also supporting a long lens with a dual support setup, constitute the complete group of solutions/improvements which together are thus far seeming to result in quite sharp results from 1/200th through 1/2. I won’t know for sure until I’m able to test a 24 to 25 ounce weight which is compact in shape, and then I’ll also need to check the rest of the shutter speed range for outliers, such as the 1/500th outlier that Lloyd found. At that point, I might have a solution. And I’ll still then need to test all the focal lengths and other lenses in my outfit at every shutter speed to be safe, and know that a particular damping mass (plus other measures) is a reliable solution.

I might also mention that at 150 mm with the Canon 70-200, I can get great results without going beyond the 16.8 oz mass of this deadweight which I already have in hand. We also can’t know just exactly how the Sony G 70-200 f/4 OSS lens will behave, once it becomes available, but I am pretty certain at this point that there is no way that simply connecting the camera and lens to a tripod, with the camera vertical, either by the lens foot or directly via the camera, will give usable images at the full range of shutter speeds. This despite the advantage of being a native lens with a firmer connection to the body, which will surely help. If I am right, this will be a big problem for Sony. But it’s impossible to know without trying it. I’m guessing that adding a deadweight of substantial mass and/or other measures such as a dual support setup, etc., will be required.

Thanks, Joe, for taking the time to fill us in or the results of your painstaking experiments. The strap lug fix for the L-bracket cantilever sounds good, but maybe a better solution, and one that would let you keep usingthe strap, wound be a shim to do the same job. OTOH, the shim could fall off and the lug won’t.

Exactly, a shim might fall off, but there may be another way. I would like the option to use a neck strap, and I’ve been thinking about wrist straps too… We can probably think of a part which can be reliably connected to the top of the L-plate which would fit, but I haven’t looked at that yet.

Today’s experiments gave me data for several more masses, and so far 23 to 24 oz is looking best, including the clamp of the deadweight but not the L-bracket. I’ve refined the position which seems optimal, such that my second-gen weight will get a screw hole which is dead center, despite that putting the center of gravity aft of the shutter itself by 3/16″ or so. At each of the interesting masses (16.8, roughly 21, 23, 24, and 26 oz.) the worst speeds were either 1/25th or 1/20th. From what we saw with Nick Wheeler’s 300 mm Nikkor, I have to suspect that his worst speed (much longer lens) may well be as slow as 1/10th or less.

So long as one weight can work OK for all of one’s lens setups that need help, this might not get too crazy for real life.

I have to say, it is fun to contemplate using tungsten instead of 316 stainless. The alloy I have in mind is commonly used for ballast and has a specific gravity of 17. Yeow!

Don, I can’t do it myself, since I don’t have Mike’s setup. You might contact him through his company’s website: Betterlight. I expect that it will look similar, but with the shutter winding and first curtain vibrations missing, at least if it is operated in EFCS mode.